Experimental and kinetic modeling study of laminar flame speed of dimethoxymethane and ammonia blends

Ammonia (NH₃) is considered a promising carbon-neutral fuel, with a high hydrogen content, that can diversify the global energy system. Blending ammonia with a highly reactive fuel is one possible strategy to enhance its combustion characteristics. Here, an investigation of blends of NH₃ and dimethoxymethane (DMM), a biofuel with high fuel-born oxygen content and no carbon–carbon bonds, is reported. Unstretched laminar burning velocity (S_L) and Markstein length of different NH₃/DMM blends were experimentally determined using spherically propagating premixed flames. The DMM mole fraction was varied from 0.2 to 0.6 while measuring S_L at 298 K, 0.1 MPa, and equivalence ratios (Φ) over the range of 0.8–1.3. The addition of DMM was found to immensely enhance the combustion characteristics of ammonia. DMM 20% (by mole fraction) in the NH₃/DMM blend increased S_L by more than a factor of 3 over neat ammonia; such enhancement was found to be comparable to 60% CH₄ in NH₃ (Φ = 0.9–1.1) blends. Increasing Φ was found to significantly decrease the burned gas Markstein length for lean cases, whereas a negligible effect was observed for rich mixtures. A composite chemical kinetic model of DMM/NH₃, aimed at interpreting the high-temperature combustion chemistry, was able to reliably predict S_L for neat NH₃ and DMM flames. Also, the predictive capability of the kinetic model to describe S_L for DMM/NH₃ blends is reasonably good. Sensitivity analysis and reaction path analysis indicated that the NH₃/DMM blends could be understood as dual oxidation processes of the individual fuels that are competing for the same radical pool.